Facile and Scalable Synthesis of Metal- and Nitrogen-Doped Carbon Nanotubes for Efficient Electrochemical CO2 Reduction

Metal- and nitrogen-doped carbon (M–N–C) is a promising material to catalyze electrochemical CO2 reduction reaction (CO2RR). However, most M–N–C catalysts in the literature require complicated synthesis procedures and produce small quantities per batch, limiting the commercialization potential. In this work, we developed a simple and scalable synthesis method to convert metal-impurity-containing commercial carbon nanotubes (CNTs) and nitrogen-containing organic precursors into M–N–C via one-step moderate-temperature (650 °C) pyrolysis without any other treatment nor the need to add metal precursors. Batches of catalysts in varied mass up to 10 g (150 mL in volume) per batch were synthesized, and repeatable catalytic performances were demonstrated. To the best of our knowledge, the 10 g batch is one of the largest batches of CO2RR catalysts synthesized in the literature while requiring minimal synthesis steps. The catalyst possessed single-atomic iron–nitrogen (Fe–N) sites, enabling a high performance of >95% CO product selectivity at a high current density of 400 mA/cm2 and high stability for 45 h at 100 mA/cm2 in a flow cell testing. The catalyst outperformed a benchmark noble-metal nanoparticle catalyst and achieved longer stability than many other reported M–N–C catalysts in the literature. The scalable and cost-effective synthesis developed in this work paves a pathway toward practical CO2RR applications. The direct utilization of metal impurities from raw CNTs for efficient catalyst synthesis with minimal treatment is a green and sustainable engineering approach.


Product Selectivity Calculation
The Faradaic efficiency (FE) of gaseous products in H-cell setup and flow cell setup at each applied potential was calculated based on the equation: Where z is the number of electrons transferred per mole of gas product (z is 2 for CO and H2), P is pressure (1.01 × 10 5 Pa), F is Faraday constant (96500 C mol −1 ), V is the gas volumetric flow rate (5.67 × 10 −7 m 3 /s), vi is the volume concentration of gas product determined by GC, R is the gas constant (8.314J/mol•K), T is the temperature (298.15K), and J is the steady-state current at each applied potential (A).
The flow rate V was measured by a bubble flow meter (Gilian Gilibrator 2) at the gas outlet in the cathode chamber to consider the potential flow rate loss from CO2 dissolving in the alkaline electrolyte.

Equipment for Materials Characterizations
Morphology, structure, and composition of the catalysts were characterized by transmission electron microscopy (TEM, FEI Tecnai G2 F20 ST), high-angle angular dark-field scanning transmission electron microscopy (Hitachi 2700C), and X-ray photoelectron spectroscopy (XPS, Omicron).The X-ray absorption spectroscopy (XAS) measurements were performed at the 12-BM beamline of the Advanced Photon Source (APS) at the Argonne National Laboratory (ANL).Note: most of the literature did not directly report one-batch mass of catalyst, however, the maximum amount of catalyst can be calculated based on the precursor composition and pyrolysis temperature since the nitrogen precursors usually completely decompose during the hightemperature pyrolysis (> 800 C) and metal contents in the form of single atoms do not contribute much to overall catalyst mass (usually less than 5 wt.%).Typically, the mass of carbon from the precursor determines the final mass of the catalyst.In Table S3, the one-batch catalyst mass is calculated based on the following conditions: (1) when the carbon precursor is carbon allotropes (e.g., CNT, carbon black, etc.), the catalyst mass is roughly equal to the mass of carbon precursor; (2) when the carbon precursor is organic materials (e.g., ZIF-8), the actual catalyst mass would be significantly smaller than the carbon precursor mass, because organic materials decompose significantly during carbonization process, and as a result, an estimated 50% mass conversion from precursor to catalyst is applied according to the literature.

Figure S4 .
Figure S4.XPS N 1s spectra and fitting of CNT-Mel synthesized at different temperature: (a) 650

Figure S5 .
Figure S5.(a) Faradaic efficiency of CO, (b) total current density, and (c) partial CO current density of CNT-Mel synthesized at different pyrolysis temperatures in H-Cell (Electrolyte: 0.5 M KHCO3).

Table S1 .
Surface nitrogen and metal concentration detected by XPS.

Table S3 .
Comparison of the mass of catalysts synthesized in one batch for high-performing Fe or Ni-based M-N-C catalysts in the literature. 36

Table S4 .
Comparison between the catalyst in this work with state-of-the-art Fe-based/Ni-based M-N-C catalysts in the flow cell when running long-term tests.